1. Field of the Invention
This invention relates to an electron beam exposure apparatus, and more particularly to an electron beam exposure apparatus for use to manufacture an integrated circuit of a semiconductor device or a mask for the formation of an integrated circuit.
2. Description of the Related Art
In recent years, much effort has been and is being directed to reduction of the dimensions of elements of a semiconductor integrated circuit in order to raise the density and the speed of operation of the semiconductor integrated circuit. In order to reduce the device dimensions, research and development for an optical exposure apparatus which makes use of ultraviolet rays have been directed to reduction in wavelength of light used, increase in NA (numerical aperture) of the lens, optical improvement of the exposure apparatus such as employment of a deformed shape light source, application of an exposure method of a new technique which employs a phase shift mask or a like element, and so forth.
Further, in parallel with such improvement of the optical exposure technique, development of new exposure methods such as electron beam exposure or X-ray exposure has proceeded. Particularly, various attempts employing electron beam exposure have been proposed for the formation of an integrated circuit having a fine pattern such as a 256 megabit DRAM.
While those electron beam exposure apparatus are classified into two types including a point beam type and a variable rectangular beam type, they both require a long period of time for exposure because a pattern is divided into unit small areas or rectangular areas and either a point beam is deflected to successively scan the unit small areas or an electron beam having a beam spot of a size corresponding to the pattern is deflected to draw the pattern with one stroke. For example, for a 256 megabit DRAM mentioned above, the exposure time required for one chip is approximately 10 minutes, which is longer by approximately 100 times or more than that required with an exposure apparatus of the optical exposure type. Further, the electron beam exposure apparatus are disadvantageous also in that they are more expensive than optical exposure apparatus.
Meanwhile, M. B. Heritage has published a report entitled "Electron-projection microfabrication system", J. Vac. Sci. Technol., Vol. 12, No. 6, November/December 1975, pp.1135-1140. The report discloses a system wherein, preparing a mask having a pattern corresponding to an entire memory chip, the entire chip is exposed by a single electron beam irradiation operation in order to reduce the exposure time mentioned above.
The system just described, however, has not been put into practical use as yet due to difficulty in implementation of an electron optical system for a large area electron beam which assures a sufficiently high degree of accuracy over the entire area of a chip of more than several millimeter square.
Meanwhile, another electron beam exposure apparatus is disclosed in Japanese Patent Laid-Open Application No. Showa 52-119185 wherein a mask having several patterns formed at different block positions thereon is provided on a path of an electron beam and deflection means is provided for selecting, on the mask, an electron beam of a diameter sufficient to allow each block to be irradiated by the electron beam. Further, an electronic optical system is disclosed in a report No. 27a-K-6 by T. Matsuzaka et al., "Study of EB cell Projection Lithography (1): Electron Optics", Extended Abstracts for the 50th Autumn Meeting of the Japan Society of Applied Physics in September, 1989, p.452, wherein a plurality of openings are provided for a second aperture while the output of a shaping deflector is increased in magnitude and a variable magnification lens as well as a fly-back deflector are provided. Furthermore, it is disclosed in another report No. 27a-K-7 by Y. Nakayama et al., "Study of EB cell Projection Lithography (II): Fabrication of Si aperture", appearing on the same page of the same publication that an aperture is formed from single crystal of Si for EB cell projection lithography and anisotropic wet etching is used for the formation of a Si thin film part of the aperture while dry etching is used for working of an opening of the aperture. Those techniques all relate to a method of transferring a pattern having some repetitions not to an entire area but to a portion of an area of a chip, and contemplates reduction of the exposure time by preparing, as a mask, a partial region of a size approximately corresponding to a beam spot having a size sufficient to assure a uniform electron flow density from within a periodical pattern group of a chip.
However, while those electron beam exposure apparatus of the partial collective transfer type are effective for a semiconductor device which includes a comparatively large number of repetitive patterns such as a DRAM, they are disadvantageous in that, where they are used to manufacture a semiconductor device having ordinary logic circuits having a comparatively small number of repetitive patterns, a long exposure time is required similarly as in the system described hereinabove wherein an electron beam is deflected to draw a pattern with one stroke.
Further, T. H. P. Chang et al. have published a report entitled "Arrayed miniature electron beam columns for high throughput sub-100 nm lithography", J. Vac. Sci. Technol., B10(6), November/December 1992, pp.2743-2748. The report proposes a system wherein, in order to reduce the exposure time described above, a substrate on which a large number of electron optical microcolumns arranged in a matrix of rows and columns are formed is prepared and used for simultaneous exposure at positions corresponding to the electron optical microcolumns by a single electron beam irradiation operation. Besides, according to the technique, if electron beam control signals corresponding to different pattern data are supplied to the individual microcolumns electron optical microcolumns each including a very small electron lens and a very small deflection system, then exposure with an arbitrary pattern can be performed at a high speed irrespective of whether or not a repetitive pattern is included.
With the technique just described, however, a very small electron lens, a very small deflection system and other elements must be formed in each of the electron optical microcolumns, and it is difficult to form those very small structures, particularly, the very small deflection systems, with sufficient degrees of working accuracy and uniformity. Accordingly, the technique is disadvantageous in that, since the accuracy in position of the electron beams by deflection is low, the accuracy in pattern formation on a specimen is low and the apparatus cost is high.
A different electron beam exposure apparatus is disclosed in Japanese Patent Laid-Open Application No. Showa 56-98827 which includes a cold cathode electron beam generator having a structure wherein electron beam generation elements are arranged one- or two-dimensionally. In the exposure apparatus, a large number of cold cathode electron beams are generated simultaneously from the electron beam generator and are accelerated by an acceleration electrode, and then a one- or two-dimensional pattern of the electron beams is projected generally in a reduced or expanded scale on a specimen by a converging lens. By increasing the number of very small spots which can be exposed simultaneously and controlling the electron beam generation elements corresponding to the respective very small spots individually in this manner, reduction of the exposure time can be achieved irrespective of whether or not a repetitive pattern is included.
In the electron beam exposure apparatus disclosed in the last-mentioned document, however, since electron beams from those of the electron beam generation elements which are located at peripheral positions of the electron beam generator correspond to positions outside an optical axis of the electron optical system, that is, since those electron beams are positioned in a spaced relationship away from the center axis of the electron optical system in one direction, they exhibit comparatively large aberrations such as coma or astigmatism. Consequently, the electron beam exposure apparatus is disadvantageous in that the accuracy in shape of a pattern on a specimen is low. Even if an electron optical system having a high degree of accuracy is used in order to eliminate the disadvantage, the accuracy in pattern shape cannot be raised to such a high level that the pattern can be practically used. Further, since electron optical columns of a scale having a height of approximately 1 m are required similarly as in conventional apparatus, the electron beam exposure apparatus is disadvantageous also in that a high cost is required.